President Barack Obama speaks at a town hall meeting in Washington. Photograph: Jim Lo Scalzo/EPA

President Barack Obama says his Russian counterpart, Vladimir Putin, should be held "directly responsible" for ensuring the site of the Malaysian aircraft downed in eastern Ukraine is made accessible to international investigators.

In pointed remarks intended to place the focus firmly on the Russian president, Obama on Monday condemned what he described as insulting treatment of bodies at the crash site by pro-Russian separatists, as well as a failure to allow investigators unfettered access to the wreckage.

“Given its direct influence over the separatists, Russia and President Putin in particular has direct responsibility to compel [the separatists] to cooperate with the investigation,” the US president said in a statement from the White House.

“The burden now is on Russia to insist that the separatists stop tampering with the evidence, grant investigators, who are already on the ground, full and unimpeded access to the crash site. The separatists and their Russian sponsors are responsible for the safety of investigators doing their work.”

The Obama administration has said all the evidence surrounding the downed Malaysia Airlines flight MH17 points towards pro-Russia separatists in eastern Ukraine having shot down the civilian aircraft on Thursday. US officials believes the separatists are likely to have used anti-aircraft systems provided by Russia and have suggested Moscow may also have trained the separatists in how to use the sophisticated weaponry.

There is growing frustration in Washington because the international investigators, who include Dutch and US experts, will be unable to provide a definitive and authoritative explanation of the crash if their work continues to be hampered. There has also been shock at the treatment of the remains of the 298 victims, which were scattered across wheat fields around the town of Grabovo, near the Russian border.

Obama said the secure recovery of bodies was “the least that decency demands” and described the reported mistreatment of body remains by pro-Russian separatists an “insult to those who have lost loved ones”.

The UN security council will vote later on Monday on an Australia-proposed resolution demanding international access to the Ukraine plane crash site and a ceasefire around the area – a measure which could result in a Russian veto.

International forensic and aviation accident experts were on the ground on Monday near the scene of the crash site, but were still unable to conduct their work without interference from armed military men who control the area.

“Unfortunately, the Russian-backed separatists who control the area continue to block the investigation,” Obama said. “They’ve repeatedly prevented international investigators from gaining full access to the wreckage. As investigators approached, they fired their weapons into the air. These separatists are removing evidence from the crash site. All of which begs the question: what exactly are they trying to hide?”

Obama added that Moscow has “extraordinary influence over these separatists”, who he said were now considered not just a danger to Ukrainians but the broader international community.

“Now is the time for President Putin and Russia pivot away from the strategy that they have been taking and get serious about trying to resolve hostilities within Ukraine,” he said.